Abstract

This paper describes a system of interferometric control as applied to the smaller ruling engine of the Mount Wilson Observatory. The usual mechanism intermittently advances the grating carriage with a spacing closely approximating an integral number of fringes of green Hg198 light. A Michelson interferometer monitors this motion. The interferometer is modulated by deflecting the compensating plate electromagnetically, thus correcting for barometric pressure changes and also causing the fringe pattern to oscillate with a small amplitude at 60 cps. The oscillating fringe pattern is scanned by a phototube and is reproduced on a monitoring cathode-ray tube; any decentering of the nth fringe is detected by synchronous demodulation and is converted to a stored electrical charge. During each spacing operation, a differential correction proportional to the stored error signal is introduced into the mechanism. Corrections average about 1 centifringe (2.5 × 10−7 cm). The system is relatively simple and ensures a very high order of precision in spacing, with extremely straight grooves.

© 1962 Optical Society of America

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References

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  1. G. R. Harrison, N. Sturgis, S. C. Baker, G. W. Stroke, J. Opt. Soc. Am. 47, 15 (1957). See alsoG. R. Harrison, G. W. Stroke, J. Opt. Soc. Am. 45, 112 (1955).
    [CrossRef]
  2. H. D. Babcock, H. W. Babcock, J. Opt. Soc. Am. 41, 776 (1951). For an account of certain developments in the ruling of gratings, see Annual Report of the Director, Mount Wilson Observatory, in the Yearbook, Carnegie Institution of Washington, No. 11 (1912) to No. 60 (1961), inclusive.
    [CrossRef]
  3. A. K. Pierce, J. Opt. Soc. Am. 47, 6 (1957).
    [CrossRef]

1957 (2)

1951 (1)

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Figures (3)

Fig. 1
Fig. 1

Schematic diagram of ruling engine with intermittent control system. The moving mirror, M2, of the Michelson interferometer is on the grating carriage. The circular fringe system, which is imaged at a, is modulated by oscillation of the tilted compensating plate. Any displacement of the chosen fringe is measured by the phototube–amplifier–demodulator section, and during the next subsequent spacing operation the appropriate differential correction is applied by the motor.

Fig. 2
Fig. 2

Computer for automatic barometric correction. With inputs from a barometer and from the grating carriage, the analogue computer delivers to the deflecting coil of the compensating plate a direct current proportional to ΔpΔs. The transformer introduces a 60 cps component for oscillating the fringe system with small amplitude.

Fig. 3
Fig. 3

The sine curve of amplitude b represents an inherent mechanical error, where the ordinates correspond to errors in the positions of numbered grooves. The filled circles indicate groove positions as corrected by the intermittent interferometric control system. The phase is shifted 90°. If there are N grooves per turn, and if the feedback coefficient is 0.7, the amplitude of the error is reduced by the factor 2.8π/N.

Tables (1)

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Table I Groove Spacing Related to Number of Fringes

Equations (6)

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1 = b sin θ 1 , 2 = b ( sin θ 2 sin θ 1 ) + ( 1 β ) 1 , 3 = b ( sin θ 3 sin θ 2 ) + ( 1 β ) 2 , n = b ( sin θ n sin θ n 1 ) + ( 1 β ) n 1 .
n = b ( sin θ n sin θ n 1 ) + ( 1 β ) b ( sin θ n 1 sin θ n 2 ) + ( 1 β ) 2 b ( sin θ n 2 sin θ n 3 ) + ( 1 β ) 3 b ( sin θ n 3 sin θ n 4 ) + .
sin θ n sin θ n 1 sin θ n 1 sin θ n 2 2 sin 1 2 ( θ n θ n 1 ) cos 1 2 ( θ n θ n 1 ) = 2 π N cos θ n .
n = 2 π b N ( 3 3 β + β 2 ) cos θ n .
n = 2.8 π b N cos θ n ,
n = 0.01 b cos θ n .

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